Safety considerations lead to the replacement of pure lithium by heavier carbonaceous materials, with the general formula LixC6, as anodes for lithium batteries.
In order to minimize the loss in energy density due to this replacement, X which is the molar ratio between 6 carbons and Li, in LixC6 must be maximized and the irreversible capacity loss (QIR) in the first charge of the battery must be minimized. The maximum value for the reversible intercalation of lithium (DX) depends on the carbon type and is typically between 0.5 (186 mAh/g carbon) for non-graphitic carbon to 1 (372 mAh/g carbon) for graphites. There is a larger variation in QIR between 30% to 90%. It was generally found that QIR is larger for graphites than for petroleum coke. Therefore in order to be able to benefit from the use of graphite, (having X=1), means should be taken to decrease its QIR. QIR is attributed to SEI (Solid Electrolyte Interphase) formation and to exfoliation of the graphite.
When a carbonaceous electrode is cathodically polarized to potentials lower than 2.0 V vs. lithium, several reactions take place: Lithium intercalation with or without co-intercalation of solvent molecules, partial or complete reduction of solvent molecules and anions, precipitation of insoluble reduction products to form a Solid Electrolyte Interphase (SEI), diffusion of partially reduced species (such as semicarbonates) from the surface of the carbon into the solution, reduction-induced polymerization of solvent molecules (such as cyclic ethers and esters). At potentials lower than 0.5V, dissolution of solvated electrons into the electrolyte takes place. The fraction or yield of each reaction depends on: type of carbon, type of salt and solvents, temperature, impurities and potential (current density). Further reactions depend on electrolyte stability. In highly purified electrolytes which consist of thermodynamically stable anions (like I) and kinetically stable solvents such as ammonia, some amines and ethers, the solvated electrons exist for long periods of time (hours). These electrolytes are for obvious reasons, not suitable for use in practical lithium batteries. In xe2x80x9cbattery-gradexe2x80x9d ether-based electrolytes, or when reducible anions such as AsF6 or reducible solvents such as PC are used, these solvated electrons immediately react to form SEI on the electrode.
Highly graphitized carbons have a large capacity, but suffer from solvent co-intercalation and degradation in performance. In ether-, PC- and MF-rich solutions, the solvent co-intercalation is a significant problem, while EC addition alleviates the problem. If reversible, it may cause disintegration (exfoliation) of the graphite crystallites as a result of large variation in the lattice spacing (large changes in d002). The large variation in dimension may cause cracks in the xe2x80x9cprotectivexe2x80x9d SEI which lead to further (and faster) electrolyte reduction and more co-intercalation. Reduction of the intercalated molecules may yield gasses (such as ethane and propane in EC and PC solutions) inside the graphite crystallites. This may cause exfoliation and cracks in the SEI. Once the co-intercalation of solvent molecules starts, it will be very difficult to prevent. Therefore it must be prevented from the outset i.e. prior to or in the first charge. We believe that high quality SEI can eliminate or effectively prevent the co-intercalation of solvent molecules and the degradation process described above.
The present invention relates to non-aqueous batteries whose anode comprise carbon-based particles bounded by appropriate binders, which binders may also be a carbon-based material, which carbonaceous particles (beads, powder, wiskers, etc.), are coated by a thin, submonolayer of up to 0.1 xcexcm solid electrolyte interphase (SEI), which film is an M+ conductor and electronic insulator and consists of alkali (M) or alkaline-earth metal (MA) salts, oxides or sulfides or a mixture of these which optionally contains up to 30% organic binder or polymers, which salts are insoluble in the battery electrolyte and which preferably are fluorides, chlorides, carbonates, semicarbonates, surface carboxylic salts or a mixture of these, which SEI is chosen to be compatible with the particular electrolyte of the battery. According to a preferred embodiment the solid electrolyte interphase is chemically bonded to the surface of the carbonaceous particles, preferable through oxygen bonds.
The carbonaceous material can be chosen amongst others from graphite and non-graphitic carbons which optionally contains up to 10% (atomic weight) additional elements chosen from the group of N,B,Al,Ca,Mg and Si.
Preferably, the carbonaceous particles are partially oxidized at a suitable temperature by a suitable gas selected from: dry or wet air, O2 or Cl2 to form very narrow, up to 1 nm in width, preferably 0.3 to 0.6 nm holes, pits, cracks or flaws, the carbon losses in this process being up to 30% of its weight, preferably 3 to 15%.
A suitable temperature range for this process in air and for nongraphitized carbons is 300 to 500xc2x0 C. and for graphitized carbons it is 450 to 640xc2x0 C., depending on carbon structure, degree of graphitization surface area and impurities. Following this oxidation process, the carbon may be treated with alkali or alkaline earth hydroxides (or basic salts, such as carbonates) to neutralize the surface acidic groups and turn them into a thin layer of surface carboxylic salts, chemically bound to the carbon surface. If not, these acidic surface groups will be neutralized (turn into lithium salt) in the first charge of the battery.
The carbonaceous based particles are preferably partially oxidized by dry or wet air or oxygen, where the temperature and time are optimized for each carbon material to obtain a maximum of surface acidic groups and minimum surface basic groups at a minimal weight loss (less than 3% weight loss) optionally followed by neutralization by alkali or alkaline metal hydroxides.
Anodes of the batteries of the invention may comprise carbon-based particles which were oxidized to form surface acidic groups by the use of oxidizing agents such as aqueous solutions of H2O2, H2SO4, HNO3, KClO4, KClO, etc., washed and neutralized by an alkali metal or alkaline earth metal hydroxide or their basic salts.
The carbon based particles can be coated by a thin, up to 2 nm layer of chemically bonded alkali metal or alkaline metal carbonate formed by:
a) first formation of surface basic group by reaction of clean surface (free of surface groups) carbon with wet air at low temperatures (up to 150xc2x0 C.);
b) reaction with an alkali or alkaline earth metal hydroxide at preferably 100-200xc2x0 C. or with MAH2 or MH to form Cxe2x80x94Oxe2x80x94M or (Cxe2x80x94(O)2xe2x80x94MA surface groups;
c) reaction with CO2 gas at room temperature to form Cxe2x80x94Oxe2x80x94CO2M or Cxe2x80x94Oxe2x80x94CO2)2MA surface groups, i.e. a thin chemically bound alkali carbonate surface film.
The SEI or a part of can be formed by a dissolution of the SEI materials or its precursors in a proper solvent or solvents mixture and casting a thin layer on the carbonaceous particles followed by a heat treatment if needed. Organic elastomer up to 30% (V/V) can be added to the casting solution. A preferred SEI can be formed by treatment of these carbon particles by an appropriate amount of aqueous solution of an alkali metal carbonate optionally with water soluble polymer such as PEO and evaporating the water to form 1-10 nm thick carbonate layer. The SEI or a part of it may be formed by chemical vapor deposition or thermal decomposition of an appropriate SEI precursors.
The invention further relates to a non-aqueous battery consists of a carbon based anode, an appropriate aprotic non-aqueous electrolyte and suitable cathode the carbon of which was formed by dehydration of a carbohydrate such as (C6O6H12), a polysaccharide, cellulose or starch with general formula (C6O5H10)n either by reaction with concentrated H2SO4 or by moderate temperature pyrolysis of these carbohydrates in an inert atmosphere. The carbon particles thus formed may be further heat treated under an inert atmosphere preferably at 400-1600xc2x0 C., and may further be processed as set out above. These carbons have a broad PXRD peak at scattering angle of 22 degrees as measured with a Cuxcex1K diffractometer.
The electrolyte may consist of one or more of ethyl carbonate, propylene carbonate, dimethylcarbonate, diethylcarbonate, and methylformate, the salt selected from one or a mixture of LiPF6, LiAsF6, CF3SO3Li,LiCIO4, LiN(CF3SO2)2 and the cathode consist of one of the groups; LiMn2O4, LiMnO2,Li1CoO2, LiNiO2, LiCox Ni(1xe2x88x92x)O2 and preferably assembled in the discharged state.
The electrolyte may consist of ionically conductive polymer electrolyte based on polyethylene oxide) polyacrilo nitrile, polyvinilidene difluoride, and polymetamethyl methacrylate polymers or their mixture, the salts described above, which polymer electrolyte may contain up to 80% (volume) solvent or mixture of solvents defined above.
The object of the present invention is to provide negative carbon based electrodes for long life, high capacity, high power, rechargeable non-aqueous batteries. The present invention provides means to decrease the irreversible capacity (QR) which is lost in the first charge of such batteries to increase the reversible capacity (QIR); to form chemically bonded and more stable SEI on the surface of the carbon particles which can better stand volume changes (of the carbon) during charge-discharge processes; and slow the rate of degradation processes at the carbon/electrolyte interface such as solvent polymerization and electrolyte reduction; to eliminate solvent cointercalation into the carbon (in cases of graphitic carbons).
As mentioned earlier, in the first charge of a carbon negative electrode cell there is an irreversible loss (QIR) of cathodic capacity to form a passivating layer (SEI) on the surface of the carbon particles. QIR of high surface area carbons may reach a value of 1000 mAh/gr (of carbon) which make this kind of carbon useless for battery applications.
To decrease QIR, it is preferable to form the SEI on the surface of the carbon particles before assembling the battery. This can be done by coating the carbon particle, before or after making the negative electrode, by a very thin layer, preferably 2-10 nm, of Li+-conducting film, which is an electronic non-conductor, and which is insoluble in and compatible with the battery electrolyte. Materials for such synthetic SEI (SSEI) are: MACO3, M2CO3, alkali semi-carbonates, MAO, M2O, MAS, M2S and alkali-and alkaline-earth metal salts of surface carboxylic groups (M=alkali metal, MA=alkaline earth metal). The preferred metal ion for the synthetic SEI is lithium or at least it should be the major constituent with up to 30% (atomic) of another M or MA cation, or their mixture.
To make this SEI more flexible, it may contain up to 30% by volume of an organic elastomer, such as a polyalkanes.
This SSEI can be formed in several ways, such as precipitation from aqueous or non-aqueous solutions, pyrolysis or CVD, of appropriate precursors, or electrochemically. A M2CO3 film can be formed by casting from aqueous solution containing some binders such as PEO and evaporating the water prior or after binding the carbon particles with appropriate binder such as polytetrafluoroethylene. The amount of M2CO3 should fit the surface area of the particles, to form preferably a 2-10 nm thick film. An additional advantage of the SSEI is the elimination of the solvent cointercalation problem leading to graphite exfoliation and destruction.
Due to volume changes during the intercalation-deintercalation of lithium (especially for graphitic carbons) it is preferred to form a chemically bonded SEI (CBSEI), i.e. not only to physically coat SSEI but to bond it chemically to the surface of the carbon particles. There are several ways of doing it. The first step (I) is to form surface groups on the carbon particles and then to bond SEI to these groups. The preferred processes are to form oxygen bonds by using either acidic or basic surface groups formed in the first step. In case of acidic surface groups Step II is to convert them into alkali or alkaline earth carbonates or surface carboxylic salts, and finally (Step III)xe2x80x94coat this thin chemically bonded SEI with the appropriate material to the final desired thickness. Another process is to replace Steps II and III by in-situ electrochemical formation of SEI at the first charge of the battery. This chemically bonded SEI generally provides better mechanical stability to the SEI reducing its peeling off or cracking during the intercalation-deintercalation processes.
Another process to form chemically bonded SEI is through basic surface groups. This takes several stages: 1) Oxidation by wet air (at 0-150xc2x0 C.), clean surface carbon particles (that were preheated in vacuum or in inert atmosphere at about 500-800xc2x0 C.); Treating it with alkali metal hydroxides to form Cxe2x80x94Oxe2x80x94M surface groups; Passing CO2 to form Cxe2x80x94Oxe2x80x94CO2M surface carbonates.
Acidic surface groups can be formed by gas phase oxidation using dry or wet air or O2 at temperatures between 250-650xc2x0 C. depending on the type of carbon. The temperature is lower for non-graphitic and higher for graphitic carbons. It is also possible to form it by wet oxidation using aqueous solution of oxidizing materials such as HNO3, concentrated H2 SO4, KClO4+acid, H2O2, etc.
Basic surface groups can be formed by reaction of clean surface carbon particles with basic aqueous solutions.
Fully graphitized carbon can be intercalated up to Li1C6 or up to 372 mAh/gr carbon. However, more lithium can be stored in crystal defects or in molecular size pits, pores, voids, cracks or flaws whose size or diameter is less than 1 nm (or preferably less than 0.6 nm). These carbons are called molecular sieve carbons. These molecular sieve carbons can be made by controlled oxidation, preferably by air or O2 (dry or wet) at temperature range of 350 to 650xc2x0 C. depending on the type of carbon. It is low for non-graphitic carbons and high for graphitic carbons. For each carbon optimum heat treatment temperature and time must be chosen, this gives a maximum increase in QR and minimum in QIR.
It is believed that during mild oxidation pore formation and expansion takes place mostly at the edges of the graphite crystallites (or at the edges of the particles). As a result, large fraction of pores will have a crack-like structure, i.e. narrow long and deep cracks. These kinds of long and narrow cracks have been observed at the graphite crystallite edge in our laboratory by using the STM technique. Another way of characterizing pore size distribution is by absorption of a variety of gas molecules having different sizes and shapes. The size of the pores increases with the increase of the temperature and time of the oxidation. Flat molecules such as benzene would preferably be absorbed in the narrow cracks, while three dimensional molecules like 1,1-dimethyl hexane or 2,2 dimethyl-propane or tetra-phenyl-methane would not. For battery applications it is preferred to form narrow pores or cracks that can absorb molecules with a diameter (or width) smaller than 0.6 nm. If the pores are large solvent molecules can penetrate into them leading to increase in QIR.
QR may be increased in another way, by using hard carbons which contain a large amount of point and plane defects. These defects may accommodate extra lithium. In general, the degree of disorder in the carbon increases as its formation (carbonization) or heat-treat temperature decreases. Low temperature carbons can be formed (at room temperature) in the dehydration reaction of carbohydrate such as sugars, starch, cotton having H to O ratio of 2:1. This can be done by using concentrated sulfuric acid or at moderate temperature pyrolysis of these materials under an inert atmosphere. After this carbonization process, the carbons can be heat treated at the desired temperature to get optimal concentration of point and plane defects or the highest QR.
During cycling the electrolyte continuously degrades. This derives from its reduction and from reduction induced solvent polymerization. It was found that an addition of small amounts of alcohols or water slowed down capacity fade. We also found that addition of MF and Li2CO3 to the electrolyte improved cycle life.